1. Field of the Invention
The present invention relates to a heat-sensitive recording material. In particular, the present invention relates to a heat-sensitive recording material containing an organic phosphate of bisphenol, its metal salt or its basic salt added thereto to improve the resistance to heat, humidity and oil in the presence of a developer during storage.
2. Description of the Prior Art
Heat-sensitive recording materials consist of a heat-sensitive color-forming layer comprising a dispersion of a sensitizer, binder and other additives in a coupling system comprising a usually colorless or light-colored coupling substance such as a leuco dye and a developer which causes coloring of the coupling substance upon heating, said layer being formed on a support such as paper, synthetic paper or resin film. When a heating element such as a thermal head or hot pen is brought into contact with the recording material in a recording device, the dye is reacted with the developer to develop a color such as black to thereby form a record.
The heat-sensitive recording materials are widely used in instrumental recorders, computers, facsimiles, telex devices, automatic passenger ticket vending machines, etc., since they are superior to other recording materials in that the records can be obtained in a short time, the noise is only slight and they are inexpensive.
As the colorless or light-colored coupling substances, for example, leuco dyes having a lactone, lactam or spiropyran ring are used. As the developers, various acidic substances have been proposed heretofore. Among them, phenolic compounds such as bisphenol A and benzyl p-hydroxybenzoate are frequently used either alone or in combination of some of them. However, when these phenols are used, the printed area is whitened or the nonprinted area is colored during the storage disadvantageously. Such a phenomenon is remarkable particularly when the recording material is stored at a high temperature or in a moist atmosphere, when a fingerprint is impressed on the print or when it is brought into contact with a polyvinyl chloride sheet. Probably this is mainly due to the influence of heat, humidity or oily substance.
It is thus quite important to improve the storage stability of the heat-sensitive recording material and attempts have been made to add various third components to improve the storage stability.
For example, Japanese Patent Laid-Open Nos. 57990/1983 and 87089/1983 proposed the addition of a specified trisphenol compound in order to improve the storability; Japanese Patent Laid Open No. 185693/1984 described that the solvent resistance was improved by using a combination of an aromatic carboxylic acid with a metal salt of an aliphatic organic acid; and Japanese Patent Laid-Open No. 39593/1984 described that a combination of a phenolic developer with a metal salt of an aliphatic carboxylic acid was ineffective in improving the storability but the storability was remarkably improved by using a specified metal benzoate. Further Japanese Patent Laid-Open No. 90284/1987 described that the long-term storability can be improved by using an organic phosphonate of bisphenol.
However, the improving effects of these processes were yet insufficient and the development of a more effective process has been eagerly demanded.